In my prior U.S. Pat. Nos. 3,845,941, 3,971,666, and 3,915,002 there are disclosed methods and apparatus for recycling asphalt-aggregate composition utilizing an elongated cylindrical drum in which the composition is heated and mixed as drum is rotated. The apparatus has a combustion chamber adjacent the drum output end, and hot gases of combustion are directed into the elongated tubes which gases are then exhausted at the opposite tube end at or adjacent the input apparatus end. The asphalt-aggregate composition to be recycled, after it has been crushed and screened is introduced into the drum at the input end, which is the cooler end of the apparatus. The drum is tilted with the input end being elevated from the output end so that the composition is drawn gravitationally to the output drum end, where it is recovered.
Heretofore, it has been the practice to introduce all of the composition particles into the apparatus at the input end, normally through a hopper, or the like, whereby all of the composition particles then pass substantially the entire drum length between the input and output drum ends as they become gradually heated and mixed. One problem with utilizing such a method in which particles of all size ranges obtained from the crushing and screening of the used asphaltic concrete material are introduced together at the drum input end is that the smaller particles often become overheated or the larger particles are not heated enough. The smaller particles traveling substantially the entire drum length and because of the relatively greater surface area and concomitantly greatest surface exposure to the heated tube surfaces may be overheated, whereas the larger composition particles, do not become so readily heated because of their greater mass:surface ratio. Due to the significant difference in heating rates of larger and smaller composition particles, in order to adequately bring the coarse particle heat temperatures up to the final desired composition temperatures, for example, of between about 200.degree. and 300.degree. F., increased residence times or increased apparatus heating, or both, for the compositions have been required. This is normally achieved by decreasing the tilt of the drum, or lowering its angle relative to horizontal, which has resulted in greater amounts and weight of composition being treated in the apparatus at any given moment. This increased weight significantly increases power requirements in rotating the large drum, which may weigh 60 tons empty, and generally puts a greater overall strain on the equipment as compared to that where composition weight in the apparatus is lower. Moreover, with the larger volumes of material being mixed in the apparatus, mixing is more difficult and less efficient. In addition, when the small particles become overheated, there results volatilization of hydrocarbon in the asphalt, as well as melting and sticking of the smaller particles to the heating tubes, which composition then builds up on the tubes, and further reduces heating efficiency of the apparatus. It is to the elimination of these problems that the present invention is directed.